1. Field of the Invention
The present invention relates to a reflection optical system. More particularly, the invention relates to a reflection reduction projection optical system for reduction projection exposure of pattern on a mask or reticle onto a wafer with coating of photo resist thereon in a process for producing a semiconductor integrated circuit.
2. Related Background Art
Conventional light exposure apparatus for production of semiconductor element such as stepper have been using near-ultraviolet light. An excimer laser and far-ultraviolet light of shorter wavelength are also used these days. As a pattern of integrated circuit is made finer, a higher resolution is desired for light exposure apparatus.
A resolving power of optical system is determined by an exposure light wavelength and a numerical aperture of optical system. As the exposure light wavelength becomes shorter, and, as the numerical aperture becomes greater, the resolving power increases. It is, however, difficult to increase the numerical aperture in view of optical designing. Also, as the numerical aperture increases, a depth of focus becomes shallower, presenting a problem in practical use.
It has been tried to use a soft X-ray or an X-ray of further shorter wavelength as the exposure light.
There have been developed various exposure systems using the X-ray. Such exposure systems are so-called proximity type to transfer a shadow pattern of mask onto a wafer. Thus the pattern of mask itself must be formed finely, which requires severe control of processing precision. Such production of mask has been so difficult.
Then promising is a reflection reduction projection optical system for transferring a reduced mask pattern onto a wafer. Shafer et al. disclose such a reflection reduction optical system in U.S. Pat. No. 4,747,678 (corresponding to Japanese Patent Application Laid-Open No. 63-163319).
The optical system is fundamentally composed of three concave mirrors and a convex mirror. A resolving power of the optical system is insufficient for production of semiconductor element due to aberrations such as spherical aberration and coma caused by the convex mirror for reduction. Addition of refractive member is also proposed in the USP to correct the aberrations. The addition of refractive member may allow excellent imaging in a circular field of view. The refractive member, however, cannot be practically used in the region of soft X-ray or of X-ray.
Another reflection reduction optical system is proposed in Japanese Patent Application Laid-open No. 63-311315, which employs an aspherical mirror for aberration correction, making the production of optical system extremely difficult.
Although the reflection reduction optical system, which can perform synchronous scan of mask and wafer at a ratio of magnification of optical system using a circular field of view, is advantageous to obtain a wide light exposure field, it has such a problem that distortion of optical system affects imaging.